An insulated DC/DC converter, specifically, a flyback type or forward type DC/DC converter is used in various power supply circuits, such as an AC/DC converter. FIG. 1 is a circuit diagram of an AC/DC converter 100r including a synchronous rectification type flyback converter 200r. 
The AC/DC converter 100r mainly includes a fuse 102, an input capacitor Ci, a filter 104, a diode rectifier circuit 106, a smoothing capacitor Cs, and the flyback converter 200r. 
A commercial AC voltage VAC is input to the filter 104 via the fuse 102 and the input capacitor Ci. The filter 104 removes noise from the commercial AC voltage VAC. The diode rectifier circuit 106 is a diode bridge circuit that full-wave rectifies the commercial AC voltage VAC. An output voltage from the diode rectifier circuit 106 is smoothed by the smoothing capacitor Cs and converted into a DC voltage VIN.
The insulated flyback converter 200r receives the DC voltage VIN at an input terminal P1, steps down the same, and supplies an output voltage VOUT stabilized to a target value to a load (not shown) connected to an output terminal P2.
A switching transistor M1 is connected to a primary winding W1 of a transformer T1, and a synchronous rectifying transistor M2 is connected to a secondary winding W2 thereof. A secondary side controller 400 switches the synchronous rectifying transistor M2 in synchronization with the switching transistor M1.
An output capacitor Co1 is connected to the output terminal P2. A feedback circuit 206 drives a light emitting element of a photocoupler 204 with a current corresponding to an error between the output voltage VOUT and its target voltage VOUT(REF). A feedback current IFB corresponding to the error flows through a light receiving element of the photocoupler 204.
A rectifier diode D2 and a smoothing capacitor Co2 form a power supply circuit 208 together with an auxiliary winding W3 of the transformer T1. A source voltage VCC generated by the power supply circuit 208 is supplied to a power (VCC) terminal of a primary side controller 300r. 
The primary side controller 300r is a quasi-resonant controller. A feedback voltage VFB corresponding to the feedback current IFB is generated at a feedback (FB) terminal of the primary side controller 300r. Further, a current detection signal VCS that is proportional to the primary current IP flowing through the switching transistor M1 is fedback to a current detection (CS) terminal of the primary side controller 300r. For the current detection signal VCS, a voltage drop of a sense resistor RS installed in series with the switching transistor M1 is used. A voltage VD generated at the auxiliary winding W3 is divided by resistors RZT1 and RZT2 and input to a zero current detection (ZT) terminal. A capacitor CZT is connected to the ZT terminal.
For example, the primary side controller 300r includes a pulse modulator in a peak current mode of a quasi-resonant mode, and generates a pulse signal SOUT having a duty ratio (or a frequency) corresponding to the feedback voltage VFB, the current detection signal VCS, and a voltage VZT of the ZT terminal to drive the switching transistor M1 connected to an output (OUT) terminal.
The present inventor has reviewed a starting operation of the DC/DC converter 200r of FIG. 1 and recognized the following technical problem.
The pulse modulator of the quasi-resonant mode is triggered to turn on the switching transistor M1 when it detects that a current IS of the secondary winding is zero (zero current).
While the switching transistor M1 is off and the current IS flows though the secondary winding W2, the voltage VD that is proportional to the output voltage VOUT is generated in the auxiliary winding W3. Further, when the current IS becomes zero, the voltage VD of the auxiliary winding W3 greatly swings in a negative direction. Here, the pulse modulator detects the zero current depending on the voltage VZT of the ZT terminal.
Specifically, after the voltage VZT of the ZT terminal exceeds a first threshold voltage VTH1 (e.g., 0.2V), when it falls below a second threshold voltage VTH2 (e.g., 0.1V) that is lower than the first threshold voltage VTH1, a bottom detection signal is asserted and the switching transistor M1 is turned on using the asserted bottom detection signal as a trigger.
However, since the output voltage VOUT is low immediately after the primary side controller 300r starts up (or when the output is short-circuited), the voltage VZT of the ZT terminal becomes low. Thus, there arises a situation where the voltage VZT of the ZT terminal cannot exceed the first threshold voltage 0.2V, which stops switching of the switching transistor M1. In order to solve this problem, when a state in which the voltage VZT of the ZT terminal is lower than the first threshold voltage VTH1 (0.2V) continues for a predetermined period of time (τ1), the switching transistor M1 is forcibly turned on.
FIG. 2 is an operational waveform diagram when the DC/DC converter 200r of FIG. 1 starts up. A vertical axis and a horizontal axis of a waveform diagram or a time chart referred to herein are properly scaled up and down for easy understanding. Each waveform shown is also simplified, exaggerated, or emphasized for easy understanding.
During an ON time of the switching transistor M1 (a high level of SOUT), the current IP of the primary winding W1 increases. When the switching transistor M1 is turned off, the current IS flows through the secondary winding W2. In a state in which the output voltage VOUT is low immediately after starting up, an OFF time TOFF becomes equal to a predetermined value τ1. Further, since the slope of the current IS during the OFF time TOFF is proportional to the output voltage VOUT, the slope is very small immediately after starting up and a decrement of the current IS per cycle is smaller than an increment of the current IP. As a result, the switching transistor M1 is turned on before a zero current where the current IS becomes zero.
When this operation is repeated, the DC/DC converter 200r starts in a continuous mode and the current IP of the primary winding W1 increases. When the DC/DC converter 200r starts in the continuous mode, a very high surge voltage exceeding 100V is generated across (between a drain and a source of) the secondary side synchronous rectifying transistor M2.